Foundation Supportworks, Inc. Case Studies: Q Street Bridge Void Fill

Tuesday, June 25th, 2019 by Ally Thomson

Challenge

Severe flooding of the Elkhorn River in Douglas County, Nebraska occurred during the spring of 2019 causing significant washouts at the Q Street bridge overpass. The high water levels created large voids behind the abutments and below the concrete approach slabs at both ends of the two lane river crossing; however, there were no signs of slab settlement as a result of the washout. The approach slabs were 10 to 12 inches thick and voids as large as about 2 feet were visible both near the abutments and at the opposite ends of the approach slabs where the concrete transitioned to asphalt pavement. Before the road and bridge could be reopened to traffic, the Douglas County Engineers required that the voids be completely filled and the approach slabs resupported, preferably with lightweight material.

Solution

The optimal remedial repair method was determined to be void filling using a combination of cellular concrete and high-strength, rigid polyurethane foam. The void areas at the pavement edges were formed up with plywood and cellular concrete placed through cored holes in the pavement. The cellular concrete had a wet cast density of 35 pounds per cubic foot (pcf) and a design 28-day compressive strength of 210 pounds per square inch (psi). PolyLevel® PL400 was then injected through the slabs to fill any remaining voids. PolyLevel PL400 is a two-part liquid urethane that is mixed and injected through small 3/8-inch drilled holes in the slab to expand into a rigid foam with a compressive strength of 70 to 80 psi and an in-place density of 4 to 6 pcf. The line of PolyLevel expanding foam products is ideal for filling voids and stabilizing concrete, or with strategic and continued injection, lift settled concrete back toward its original elevation. Approximately 85 cubic yards of low-density cellular concrete and 250 pounds of PolyLevel® PL400 were placed under the slabs. Thrasher Commercial Group (TCG) completed the work in three days; forms were set on the first day, cellular concrete was placed on the second day, and smaller voids were filled with PolyLevel on the third day. The project was completed within two weeks of Douglas County initially contacting TCG to provide a solution.

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